Running Vacuum in the expanding Universe: a unified QFT paradigm for Inflation and Dark Energy

Abstract

The concordance $Λ$CDM model, based on a rigid $Λ$-term for the entire cosmic history, has been in crisis for a long time. In our expanding Universe, an evolving $Λ$ with the expansion is intuitively much more reasonable. In the running vacuum model (RVM) framework, based on quantum field theory (QFT) in curved spacetime, quantum effects induce a vacuum energy density (VED) $ρ_{\rm vac}=Λ/(8πG)$ which is a function of the Hubble rate $H$ and its time derivatives, $ρ_{\rm vac}=ρ_{\rm vac}(H, \dot{H},\ddot{H},\dots)$. Currently, $ρ_{\rm vac}$ evolves very slowly with the expansion, $δρ_{\rm vac}\sim {\cal O}(m_{Pl} ^2 H^2)$, and this fact provides a possible fundamental origin of dark energy (DE), conceived as dynamical vacuum energy. In the RVM, Newton's $G$ is also evolving, but much more slowly (logarithmically with $H$): $G=G(\ln H)$. In the very early universe, the vacuum fluctuations induce higher (even) powers, e.g. $\sim H^4$, capable of triggering fast inflation in a very short period, in which $H$ is very large and approximately constant. This is the mechanism of `RVM-inflation'. It does not require an `inflaton' field since inflation is brought about by pure QFT effects on the dynamical background. It differs from Starobinsky's inflation, where $H$ is never constant. Furthermore, the dynamics of $ρ_{\rm vac}(H)$ and $G(H)$ can also have implications on the frequently discussed possibility that the fundamental `constants' of Nature can be mildly evolving with the cosmic expansion. Putting things together, a unified QFT framework of dark energy and inflation ensues as a realistic theory for the description of the universe as a whole on fundamental grounds. In it, dynamical VED is predicted and is much welcomed, since it fits in with current DESI measurements, preferring dynamical DE over a rigid $Λ$ term.